1. Field of the Invention
The present invention relates to the methods of concentrating the number of airborne particles and microorganisms in the air or gas medium in which they are suspended. In particular, the present invention relates to the method of concentrating aerosol particles by injecting them into a swirling air flow and transferring them to a minor air flow while the major air flow swirls away from the region of particle transfer.
2. Description of the Related Art
Airborne particles and microorganisms are concentrated in air when they are present in such small quantities that they do not yield a sufficiently strong signal when exposed to a given sensing method. For instance, when collected on a filter, there may not be enough particles or microorganisms per unit filter area. Particles or microorganisms suspended in air are herein referred to as "aerosol particles."
The principal method by which airborne particles are concentrated in air is by "virtual impaction." The term "virtual" is used in contrast to "solid-plate." In a "solid-plate impactor," the aerosol flow is directed towards a solid plate so that the high-inertia particles move towards the plate and are removed by it while the low-inertia particles continue with the laterally deflected air flow. In a "virtual impactor," the solid plate is replaced by an air interface (a "virtual" plate) so that the particles are inertially impacted into a dead air volume. Since turbulence in the dead air volume eventually washes the particles back out, unless they have deposited onto the containing wall of the dead volume chamber, some air is usually withdrawn continuously from this air space. The air flow from this receiving volume is typically between 1/5th to 1/20th of the total incoming air flow. The remaining air flow is laterally deflected.
This method is also called "dichotomous flow method," because two flows leave the interaction region while only one air flow enters it. The larger particles are transferred to the smaller flow by inertial impaction. The receiving tube usually has an inner diameter that is equal to or slightly larger than the inlet tube, and the tubes are axially aligned with each other. There are usually high particle losses to the inner surfaces for particle sizes at or near the "cut size" of this method. The "cut size" for the particle size distribution is the particle size at which the number of particles in the smaller air flow equals the number of particles in the remaining air flow. The particle losses may be high because particles that do not have enough inertia to be projected into the smaller air flow may have too much inertia to negotiate the sideways deflection with the remaining air flow.
Another method that relates to the present invention is that of aerosol particle removal by centrifugal force. The embodiment of this method is usually referred to as a "cyclone." In a cyclone, the aerosol is drawn into a cylindrical chamber so that the air makes one or more rotations inside before leaving the chamber through a tube at its center. Particles with sufficient inertia move centrifugally toward the inner wall. The problem with this method is that particles that enter the cyclone near its inner effluent tube have a great radial distance to traverse. This method is, therefore, not effective for particles less than a few micrometers in diameter. Also, it does not provide a sharp particle size separation between the collected and uncollected particles. This method does not concentrate aerosol particles in the airborne state but removes them from the air flow.